This invention relates to a method and apparatus to protectively install a transmission cable in a duct, and more particularly, to providing one or more inner ducts which embody a construction to facilitate installation in a pre-existing duct for protectively receiving a transmission cable.
It is common practice to replace sheathed cables, particularly telephone communication cable in existing ducts. Reuse of these ducts greatly reduces the cable placement operations, particularly where the existing cable straverse obstacles whether manmade or natural along a buried, submarine and/or aerial route. The use of existing ducts is also desirable where the route of the ducts is beneath city streets, presidential or business property. To avoid the costs and time to expose a duct and cable along a route, an existing cable can be pulled by a winch line from the duct. The replacement cable may embody improved technology, particularly the use of optical fibers for transmission of communication signals, hereinafter referred to as light-guide cable. When cable having metal conductors is replaced with light-guide cables, considerably less space in the duct is occupied by the replacement cable because the diameter thereof is much less as compared with the diameter of the cable having metal conductors.
Light-guide cable can be placed in the sheathing of an existing cable after the conductors are removed from the sheathing while the existing cable remains in situ. A method for removing the conductors from the cable in this manner is disclosed in my prior U.S. Pat. No. 4,197,628 and provides for forcing lubricant through a sleeve which is attached to one end of the sheathing and allowing the lubricant to flow along the conductors in the cable. After the lubricant disperses in the sheathing, the conductors are withdrawn by applying a force to one end of the conductor. Light-guide cable may then be reintroduced into the sheathing. The placement operation of light-guide cable into an existing duct which may include placing the cable in the sheathing from an existing cable, requires special procedures to avoid damage to the light-guide cable. In my copending application Ser. Nos. 346,386 and 405,385, there is disclosed a method and apparatus to measure tension imposed on a cable by a pull line in a conduit which includes a tension meter coupled between the cable and the pull line. The tension meter includes a load cell connected to electronic circuitry for delivering a tension signal which is detected and used to control tension imposed on the cable. The use of the tension meter enables accurate control of the tension on the pull line, particularly when the pull line contacts the conduit or duct walls at bends which restrain the pull line and then after the cable is drawn beyond the bend, friction of the cable against the duct walls acts to resist movement of the cable. The strands of optical fiber in the light-guide cable are particularly susceptible to fatigue fractures due to an excursion of tension stress beyond a predetermined level, for example, about 500 pounds. Should the tension stresses exceed the predetermined level or even the yield point of the optical fiber material, the fibers may fracture, thus losing integrity and, therefore, usefulness.